Copolyester compositions comprising poly(ethylene naphthalate bibenzoate) biaxially oriented copolyester films

ABSTRACT

A copolyester composition comprising poly(ethylene naphthalate bibenzoate) (&#34;PENBB&#34;) and 0.001 to 5% by weight of fine silica particles having an average diameter of 0.01 to 3.0 μm. Further, films are made from this copolyester composition.

BACKGROUND OF THE INVENTION

The present invention relates to PENBB compositions, a process forpreparing the poly(ethylene naphthalate bibenzoate (hereinafter "PENBB")compositions, and biaxially oriented PENBB films obtained from thecompositions. More particularly, the present invention relates to PENBBcompositions of excellent particle dispersion in PENBB and otherproperties, and films obtained from such compositions.

Polyester films, especially biaxially stretched polyester films such aspolyethylene terephthalate (PET) films have good physical, chemical andthermal properties and are widely used as fibers, films and other moldedarticles. It is, however, desirable to improve some of these propertiesto provide still more useful film products, such as magnetic recordingmedia and the like.

A film property which is generally desired in the production of thesefilms and in handling of the produced films is good slipperiness betweenthe films themselves or between the film and other bate material. Poorslipperiness of film in the production process or post-treatment such ascoating and layer deposition or in handling of the product itself maylead to bad defects such as wrinkles or even the tendency to break andreduced commercial value of the products.

For improving slipping properties of film, a general practice is to addfine particles into polyester to provide proper roughness to the filmsurface.

As the fine particles used for such purpose, the particles of inorganiccompounds inert to polyesters such as talc, kaolin, silica, calciumcarbonate, titanium dioxide, graphite, carbon black and the like havebeen employed. However, with these inorganic particles, which areusually obtained by crushing natural minerals or by synthesis, it isdifficult to avoid the presence of some amount of coarse particles oragglomerates of particles.

If coarse particles are contained in polyester or dispersion ofparticles is poor, the filters used in the extrusion step tend to clogor the film may even break in production. Also, defects such asfish-eyes would be present in the film, giving rise to the problems suchas drop-outs in magnetic tapes or reduced dielectric strength incapacitors, etc.

Particularly in the field of magnetic tapes, requests for the reductionof size, longer play times and higher image quality are rising. For theattainment thereof, it is necessary to make the roughness on the filmsurface more uniform and finer. Especially in order to increase playingtime it is necessary to further decrease the film thickness, which inturn requires the mechanical strength, especially the stiffness(modulus) of the film to be improved.

To increase uniformity, it has been proposed to remove coarse particlesfrom the inert inorganic particles by crushing and classifying theparticles before use. For example, a method is known in which a powderobtained by crushing a mineral starting material or a powder obtainedfrom a synthesis. The crushing treatment and classifying treatment maybe either dry or wet. This method however, although capable of removingcoarse particles to a certain extent, was still unsatisfactory.

Various types of silica particles differing in average diameter arecommercially available. However, when films were made by incorporatingsuch silica particles in polyester, the produced films would fail tomeet said strict property requirements.

Generally, the finely divided particles having a relatively high surfaceactivity such as silica particles tend to form agglomerate particles andare difficult to disperse thoroughly in the copolyester monomers orpolymer. Also, agglomeration of particles accelerates in the course ofpreparation of polyester after the addition of the fine particles and,as a result, the coarse agglomerate particles are formed in thepolyester, which make the film surface roughness non-uniform, resultingin a reduced commercial value of the film. Especially in the case thatsuch film is used as support for high quality magnetic tapes such asdeposited tapes or metal tapes, a decreased output or increased signaldrop-out would result.

It has now been found that the use of the copolyester PENBB as filmmaterial while incorporating amorphous fine spherical silica particlesprovides a film of particular usefulness. The PENBB provides theimproved mechanical stiffness (modulus) and strength required to allowreduction of film thickness, while also providing improved dimensionalstability, which further improves the quality of, i.e. tapesmanufactured from the film. The incorporation of the amorphous finespherical silica particles provides the film with the desired runningproperties. The use of such particles is disclosed for PET film inEuropean Patent Application 0 257 611. Such amorphous fine sphericalsilica particles are obtained by subjecting an alkoxysilane tohydrolysis and a condensation using a specific catalyst and possess avery narrow particle size distribution, contain no coarse particles,have excellent dispersibility in PENBB, and can provide a PENBBcomposition which can be made into a film having a fine and uniformsurface roughness with minimized coarse protuberances, while also havingexcellent flatness and slip characteristics.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a copolyester composition comprisingPENBB and 0.001 to 5% by weight, based on said copolyester, of fineparticles (A), spherical silica particles obtained by subjecting analkoxysilane to hydrolysis and a condensation reaction, said silicaparticles being substantially amorphous, said fine particles having anaverage diameter of 0.01 to 3.0 μm, and the diameter distribution indexof said particles represented by the formula (1) being in the range of1.1 to 2.7: ##EQU1## wherein D₁₀ is the diameter of the particle whenthe cumulative number thereof is 10% of the zonal number of saidparticles, D₉₀ is the diameter of the particle when the cumulativenumber thereof is 90% of the total number of said particles, and D₁₀ andD₉₀ are measured under an electron microscope, said cumulative numbersbeing calculated beginning with the largest particle size.

U.S. Pat. No. 3,008,934 discloses copolyesters containing as acidderived units 4,4'-bibenzoate and a host of other dicarboxylatesincluding 2,6-naphthalic dicarboxylate. It also discloses orientedfibers and films prepared from these copolyesters, however, biaxiallyoriented PENBB films are not disclosed or envisioned. In particular,those films with improved stiffness (tensile modulus) and tensilestrength in both MD and TD as well as thermostability, UV stability,hydrophobicity, dimensional stability and impermeability toward gases incomparison to PET film are not disclosed in U.S. Pat. No. 3,008,934.

Further, the present invention relates to a process for producing thecopolyester composition and to films containing the copolyestercomposition.

The copolyester used in the present invention is PENBB. PENBB asmentioned hereinbefore is a copolyester containing as acid-derived unitat least 5 mole % of a radical of the formula ##STR1##

In the case that more than 10 mole percent of terephthalic acid derivedradicals are present in the copolymer, the content of bibenzoate derivedunits is at least 25 mole percent. Films of these copolyesters arementioned in the published German Patent Application P 4224161.8, whichis incorporated herein by reference. Preferably PENBB is a copolyesterwherein at least 80 mole percent of the acid derived units (NBB) consistof bibenzoate (20 to 80 mole percent, preferably 40 to 60 mole percent)and naphthalate (80 to 20 mole percent, preferably 60 to 40 molepercent). The remaining 20 or less mole percent may consist of otheracid derived units, which e.g. affect the melting point or thecrystallization kinetics. Preferably at least 80 mole percent of thediol-derived units consist of --O(CH₂)₂ --O-units. The remaining 20 orless mole percent consist of other diol-derived units, which e.g. mayalso affect the melting point or the crystallization kinetics. It mayalso be desirable to replace minor amounts of the acid- and/ordiol-derived units with hydroxycarboxylic-acid-derived units, e.g. suchderived from p-hydroxybenzoic acid.

In order to achieve the desired mechanical properties in the biaxiallyoriented PENBB film it is recommended that the IV value (inherentviscosity, as measured in a 1:1 weight-ratio mixture ofpentafluorophenol and hexafluoroisopropanol at a concentration of 0.2g/dl and a temperature of 25° C.) of the PENBB polymer after extrusionbe >0.5 dl/g and preferably >0.55 dl/g.

In the present invention, the particles incorporated in the PENBB forimproving the surface flatness and slipping properties of the producedfilm are fine spherical silica particles having an average diameter of0.01 to 3.0 μm. The fine spherical silica particles used in the presentinvention can be obtained by subjecting the starting materialalkoxysilane to hydrolysis and a condensation reaction by using an aminetype catalyst.

The alkoxysilane used as starting material for the preparation of thefine spherical silica particles may be selected from the alkoxysilanecompounds represented by the formula (C_(n) H_(2n+1) O)₄ Si (wherein nis an integer of 1 to 8), the preferred examples thereof beingtetramethoxysilane, tetraethoxysilane, tetrapropoxysilane,tetrabutoxysilane and the like. As the catalyst for the hydrolysis andcondensation reactions, ammonia, trimethylamine, tetraethylammonium saltaqueous solution, urea and the like, may be employed but an aqueoussolution of ammonia is preferred.

The fine spherical silica particles of the present invention can besynthesized, for instance, in the following way. A solution is preparedby dissolving tetramethoxysilane in an alcohol such as methanol(solution A). There is also separately prepared a solution by dissolvingwater and an aqueous ammonia solution in methanol (solution B). Bothsolutions are maintained at a predetermined temperature, for example,20°-30° C. Solution A is added to solution B under vigorous stirring andthe mixture is maintained at a predetermined temperature for severalhours, then centrifuged, washed with the solvent alcohol, dried anddispersed in ethylene glycol. It is also possible to employ a method inwhich ethylene glycol is added to a suspension of fine spherical silicaparticles obtained after the described reactions, and the mixturesubjected to distillation to remove excess ammonia, alcohol and water.

In any case, it is preferable that the fine spherical silica particlesbe ultimately prepared as a slurry in ethylene glycol.

The concentration of thus prepared fine spherical silica particles inethylene glycol is preferably not higher than 20% by weight, morepreferably not higher than 10% by weight, particularly preferably nothigher than 5% by weight but not less than 0.5% by weight. If theconcentration of the silica particles exceeds 20% by weight, theparticles may agglomerate when they are added in the course of thepolycondensation reaction. On the other hand, if the particleconcentration is less than 0.5% by weight, more ethylene glycol is usedthan is actually required, resulting in added expense.

The characteristic features of the fine silica particles according tothe present invention obtained from the hydrolysis and condensationreactions of alkoxysilane are that these particles are spherical inshape and extremely uniform in size. To evaluate the particle sizedistribution of the fine spherical silica particles of the presentinvention, the particle size distribution of the particles wasdetermined by electron microphotograph and the number of the particleshaving diameters were cumulated, according to size, beginning with thelargest particle size, and the diameters of the particles where thecumulative amount thereof are 10% and 90% of the whole were representedby D₁₀ and D₉₀, respectively. In the present invention, the value of D₁₀/D₉₀ is preferably in the range of 1.1 to 2.7, more preferably 1.1 to2.5, particularly preferably 1.1 to 2.3. A D₁₀ /D₉₀ value of greaterthan 2.7 signifies an excessively broad particle size distribution, inwhich case the control of film surface roughness becomes difficult orthe coarse particles increase in number. It is difficult to obtainparticles whose D₁₀ /D₉₀ value is less than 1.1 in an industrialprocess.

The fine spherical silica particles used in the present invention arepreferably 0.01 to 3.0 μm in diameter. If the diameter of theseparticles is less than 0.01 μm slipping properties of the film are notimproved. If the diameter of the particles exceeds 3.0 μm the producedfilm will be too rough and of poor quality due to the presence of theparticles with large diameters.

The fine spherical silica particles preferably comprise 90% by weight ormore of silicon dioxide and it is preferable that the volumetric shapeparameter (φv) thereof satisfies the following formula (2): ##EQU2##wherein φv is defined as φv =V/D³, V is the volume of particle (μm³) andD is maximum projected diameter (μm) of the projection of the particles.

It is also desirable that the fine spherical silica particles of thepresent invention have a specific surface area ratio (R), defined byformula (3), of 5.0 or greater, preferably 10 to 100: ##EQU3##

The "specific surface area ratio" indicates the degree of porosity ofthe silica particles. When the value of this ratio is less than 5, theparticle has poor affinity for PENBB and also is unsatisfactory forimproving the wear resistance of the produced film. Also, when the ratioexceeds 100, the interaction between the particles promotes theagglomeration of the particles, which reduces the performance of theslurry filter and gives rise to coarse protuberances on the filmsurface.

In the present invention, the content of the fine spherical silicaparticles in the PENBB composition should be 0.001 to 5% by weight basedon PENBB. When the particle content is less than 0.001% by weight,slipping properties and wear resistance of the film are not sufficientlyimproved, and when the particle content exceeds 5% by weight, the filmsurface is too rough due to increased amounts of agglomerates ofparticles and coarse protuberances increase.

The fine spherical silica particles used in the present invention have avery narrow particle size distribution and excellent dispersibility inethylene glycol, so that no specific dispersing or classifying treatmentis necessary and also can be easily filtered with a slurry filter.

In the preparation of the PENBB composition of the present invention,the fine spherical silica particles are preferably added in the courseof the PENBB synthesis. For example, it is recommended to add theparticles before the ester exchange reaction, before the esterificationreaction, during the ester exchange or esterification reaction, or afterthe end of ester exchange or esterification reaction and before start ofpolycondensation reaction.

The copolyester is obtained by polycondensation of the correspondingdiacid mixture or lower dialkyl diester and the corresponding diol. Bothcomponents should preferably be employed in roughly equimolar ratios. Itmay however be advantageous to employ one of the components--especiallythe diol--in excess, for instance in order to influence the reactionkinetics or to serve as a solvent. The polycondensation is carried outaccording to known processes used e.g. in the production of polyethyleneterephthalate (PET). Usually about 100 mole-% of the dicarboxylic acidor dialkyldicarboxylate mixture are mixed with >100 mole-% of thecorresponding diol(s). This mixture is then heated to about 200° C.,preferably in the presence of a transesterification catalyst, untilsufficient lower alkyl alcohol has been removed from the mixture viadistillation. This reaction yields an oligomer or a low molecular weightpolyester, which is subsequently subjected to polycondensation,preferably in the presence of a stabilizer and/or catalyst. Usefulstabilizers and catalysts can be polyphosphates, triorganyl phosphates,antimony trioxide or tetraalkoxy titanate(IV or mixtures oftriphenylphosphate and antimony trioxide. A preferred process for theproduction of such copolyesters is described in U.S. patent applicationSer. No. 07/735,553 which is incorporated herein by reference, and inU.S. Pat. No. 5,453,321, which issued from a continuation-in-part of acontinuation of said application Ser. No. 07/735,553 (which earlierapplications are now abandoned). A further increase in molecular weightcan be achieved by solid phase polycondensation at a temperature justbelow the melting point under vacuum, or a stream of dry air or inertgas.

In the copolyester composition of the present invention, the use ofother types of inactive particles in combination with the fine sphericalsilica particles (particles (A)) is preferred. This further improveswear resistance of the film made from the PENBB composition. This alsoimproves the electromagnetic properties of the film, especially when itis used for a magnetic tape.

The following two groups of particles (I and II) can be cited as typicalexamples of other types of inactive particles (particles (B)) usable inthe present invention:

B-I: silica particles other than the fine spherical silica particles,calcium carbonate particles, amorphous zeolite particles, fine particlesof anatase type titanium dioxide, calcium phosphate kaolin, talc, clayand the like;

B-II: fine particles precipitated by the result of the reaction ofphosphorus and compounds and residues of ester exchange catalysts in thecopolyester condensation reaction system, for example, fine particlescomprising calcium, lithium and phosphorus compounds, fine particlescomprising calcium and phosphorus compounds, and fine particlescomprising calcium, magnesium and phosphorus compounds.

In the case that other types of inactive particles (particles(B)) areused in combination with particles (A), the mixing ratio, of particles(A) and particles (B-I) or particles (B-II) is preferably selected inaccordance with the following condi- tions.

In the case of combined use of particles (A) and particles (B-I) theratio of diameter of (B-I) particles to diameter of (A) particles (D₂/D₁) is 1.1-3, preferably 1.5-2.0, and the amount of (B-I) particlesused is 0.005 to 1.0% by weight, preferably 0.01 to 0.7% by weight,based on PENBB.

If the D₂ /D₁ ratio is less than 1.1, the improvement of wear resistanceand winding properties of the film made from the composition will beunsatisfactory. Also, if the D₂ /D₁ ratio exceeds 3.0, the surface ofthe film will become too rough and also the amount of large particles inthe film, which cause drop outs or other defects, will further reducethe film quality.

If the content of particles (B-I) in the copolyester composition is lessthan 0.005% by weight, the wear resistance and winding property of thefilm will not be improved to satisfactory degree. If the content ofparticles (B-I) exceeds 1.0% by weight, there will result too largesurface roughness of the film and an increased amount of large-sizedparticles which may cause drop-outs or other defects, thus deterioratingthe film quality.

The weight ratio of particles (B-I) to particles (A) is preferably inthe range of 0.005 to 0.5, more preferably 0.01 to 0.1.

It is possible to use particles (A) themselves as particles (B-I) as faras the particle size and amount of these particles fall within theabove-defined ranges.

In the combined use of particles (A) and particles (B-II), the latterare used in an amount of 0.05 to 1.0% by weight, preferably 0.05 to 0.4%by weight, based on PENBB. If the content of (B-II) particles is lessthan 0.05% by weight, the surface roughness of the film will be too lowto provide the desired slipping properties. On the other hand, if thecontent of (B-II) particles is higher than 1.0% by weight, the filmsurface will be too rough, causing a deterioration of electromagneticproperties when in use as an electromagnetic recording medium substrate.The diameter of particles (B-II) is usually about 0.1 to 3 μm.

A preparation process of PENBB containing precipitated particlesespecially suited for use in this invention will now be described indetail.

A copolyester containing such particles can be obtained, for example, bycarrying out an ester exchange reaction in the presence of a lithiumcompound and a calcium compound, adding to the reaction product at leastone compound selected from the group consisting of phosphoric acid,phosphorous acid or alkyl or allyl esters thereof in an amount of 0.6 to3 times the total amount of said metallic compounds, and successivelycarrying out a polycondensation reaction.

Lithium compounds which can be dissolved in the esterification or esterexchange reaction product can be used in the ester exchange reaction.Examples of such lithium compounds are salts of aliphatic carboxylicacids such as acetic acid, propionic acid and butyric acid, salts ofaromatic carboxylic acids such as benzoic acid and p-methylbenzoic acid,and lithium glycolates of ethylene glycol or propylene glycol. Amongthem, aliphatic lithium carboxylate, especially lithium acetate, ispreferred. The amount of such lithium compounds added is preferably 0.03to 0.4% by mole, more preferably 0.1 to 0.2% by mole, based on thearomatic dicarboxylic acid components for PENBB.

As the calcium compounds which can be used in the ester exchangereaction, it is possible to use any of those calcium compounds solublein the esterification or ester exchange reaction product. Examples ofsuch calcium compounds are salts of aliphatic carboxylic acids such asacetic acid, propionic acid and butyric acid, salts of aromaticcarboxylic acids such as benzoic acid and p-methylbenzoic acid, andcalcium glycolates of ethylene glycol or propylene glycol. Among them,aliphatic calcium carboxylate, especially calcium acetate is preferred.

The amount of such calcium compound to be used in the ester exchangereaction is preferably 0.05 to 0.3% mole-%, more preferably 0.08 to0.15% mole-% based on the aromatic dicarboxylic acid components ofPENBB.

As the phosphorus compound, any phosphorus compounds which have beenreacted with the metallic compounds and are partially or whollyconverted into a phosphate or phosphite salts may be employed, but it ispreferred to use phosphoric acid, trialkyl esters of phosphoric acid,partial alkyl esters of phosphoric acid, phosphorous acid, trialkylesters of phosphorous acid, and partial alkyl esters of phosphorousacid.

Such phosphorus compound is added in an amount of 0.6 to 3 equivalents,preferably 0.8 to 2 equivalents, more preferably 0.9 to 1.5 equivalentsto the total amount of the metallic compounds.

The equivalent ratio of phosphorus compound to metallic compounds isgiven by the following formula (4): ##EQU4## wherein P, Ca and Lirepresent the amount of phosphorus compound, calcium compound andlithium compound (expressed in moles), respectively.

As described above, precipitated particles containing metallic elementsand phosphorus element can be obtained by using the metallic compoundsand phosphorus compounds in combination.

It is especially preferable that each of calcium, lithium and phosphoruselement be contained in an amount of 1.0% by weight or more based on theprecipitated particles. In this case, it is possible to form a greatmany of sharp and uniform fine protuberances on the film surface, whichgreatly contributes to the improvement of slip characteristics of thefilm.

Of course, part or all of lithium or calcium compound may be added afterthe ester exchange reaction has been completed.

In this invention, the particles containing lithium, calcium andphosphorus elements precipitated in PENBB are used in an amount of 0.05to 1.0% by weight, preferably 0.05 to 0.4% by weight, more preferably0.06 to 0.18% by weight, based on the copolyester film. In this case,the quantitative determination of the precipitated particles is made inthe following way.

Quantitative determination of precipitated particles in the PENBB film:

1.0 liter of o-chlorophenol is added to 100 g of PENBB, and the mixtureis heated at 120° C. for 3 hours and then centrifuged with BeckmannUltracentrifuge L3-50 at 30,000 r.p.m. for 40 minutes, and the resultingparticles are dried in vacuo at 100° C. These particles are measured bya differential scanning calorimeter, and when a melt peak correspondingto PENBB has appeared, the particles are added with o-chlorophenol,heated, then cooled and centrifuged again. When the melt peak due toPENBB has disappeared, the weight of said particles is regarded as theweight of precipitated particles and the weight ratio to PENBB iscalculated.

In the present invention, when using B-I particles, it is preferable tochoose those with a Moh's hardness of 4.0 or below. If the Moh'shardness of the particles is higher than 4.0, the film and rolls maysuffer from scratches when the film is wound up or subjected tocalendaring after it has been coated with a magnetic layer.

The PENBB composition of this invention can be used for producing abiaxially drawn PENBB film. To produce the film, the polymer melt isextruded through a die onto a chill roll where it solidifies, is thenbiaxially oriented, heat set, optionally post treated and wound on aroll. For a multilayer film, known methods for coextrusion, in-line oroff-line coating can be used. The solidified film as extruded on thechill roll should be obtained in an essentially amorphous state. Toachieve this, the melt film must be pinned to the chill roll by a knownmethod such as electrostatic pinning or vacuum, air knife or the like.

The biaxial orientation of the film is achieved by stretching the filmat elevated temperature in the machine (MD) and transverse direction(TD). This stretching can be either simultaneous or sequential. In thecase of sequential stretching the first stretching step can be in eitherMD or TD, followed by stretching in the other direction. The orientationin MD can also be achieved in several steps, either one after anotherprior to stretching in TD, or before and after the TD stretching.Preferred temperatures for stretching lie between the glass transitiontemperature and about 30° C. above the cold crystallization temperatureof the PENBB copolymer composition in use (both temperatures can easilybe measured on amorphous films by DSC). The total stretch ratios (λ) inMD and TD lie between 1:2 and 1:10, preferably between 1:2.5 and 1:5.The product of the total stretch ratios should be between 1 to 30preferably between 5 to 20. Biaxial drawing is performed such that thebirefringence is 0.2, preferably <0.1 to ensure adequately isotropicproperties. Birefringence as mentioned herein is the absolute value ofthe difference between the maximum and minimum refractive indices in theplane of the film, as measured on common instruments such as Abberefractometer, optical bench or compensators.

In order to optimize properties, relaxation steps can be included in theorientation and heat setting processes.

The heat setting takes place at a temperature between the coldcrystallization temperature and the melt temperature of the copolymercomposition.

In some cases a surface treatment such as corona, plasma or flametreatment should be employed before winding the film on a roll.

The biaxially oriented PENBB film obtained from the copolyestercomposition of the present invention is very useful as base film formagnetic recording media such as magnetic tapes, floppy discs, etc. oras base film for various other types of commercial products such ascapacitors, photographic plates, electric insulators, thermosensitivetransfer sheets, package films, etc.

The biaxially oriented PENBB film obtained from the copolyestercomposition of the present invention usually has an average surfaceroughness (R_(a)) of 0.003 to 0.29 μm and a thickness of 1 to 400 μm,particularly 1 to 20 μm.

Also, the b,axially oriented copolyester film obtained from the PENBBcomposition of the present invention is excellent in wear resistance andrarely produces drop-outs in magnetic tapes made from it, as the filmcontains the specific fine spherical silica particles described above.These quality characteristics are particularly pronounced in the filmwhich satisfies the requirements expressed by the following formulae (5)and (6) at the same time:

    ΣH.sub.n ≦20, 7≧n≧3             (5) ##EQU5## wherein H.sub.n is the number of interference fringes of n-th order (fringes/cm.sup.2) when the film surface was measured by double beam interference method.

The use of the biaxially oriented PENBB film of the present inventionfor magnetic recording media and capacitors will be described in detail.

In use of the biaxially oriented PENBB of the composition of the presentinvention as a magnetic recording medium, the starting PENBB compositionis preferably the one whose volume resistivity (P_(v)) in the moltenstate is 1.0×10⁸ Ω.cm or below. If the volume resistivity of thestarting PENBB composition when melted exceeds the above value, smalldents or recesses are formed in the amorphous sheet made from thecomposition by an electrostatic cooling method, and when a biaxiallystretched film is made therefrom the non-uniformity of the filmthickness increases, causing non-uniformity of the magnetic coatings,resulting in a deterioration of electro-magnetic transducingcharacteristics of the magnetic recording medium.

The volume resistivity (P_(v)) of the starting PENBB composition can belimited to 1.0×10⁸ Ω.cm or below by, for example, adjusting the ratio ofthe phosphorus compound used as thermal stabilizer to the alkaline earthmetal compound used as ester exchange catalyst (P/Me), or by adding analkaline metal compound after the end of ester exchange reaction or inthe early or middle stage of polymerization reaction.

Generally, magnetic recording media can be roughly classified into twotypes: magnetic tapes and magnetic discs. The thickness of base filmused for the former is usually 4 to 30 μm and that used for the latteris usually 30 to 100 μm.

An important mechanical property required for the film for magneticrecording media is that the F₅ value of the film in its machinedirection is not less than 120 MPa, preferably not less than 160 MPa.The film with an F₅ value of less than 120 MPa may tend to elongateafter tension, so that the advantageous properties of the fine sphericalsilica particles can not be put to best use.

The effect of the present invention is maximized when the F₅ value inthe machine direction of the film (F_(5MD)) and the F₅ value in thetransverse direction (F_(5TD)) are defined by the following formula (7),especially by the formula (8): ##EQU6##

Conventional high-strength PET films falling in the above range of F₅values had problems such as anisotropy of physical properties in thetransverse direction or insufficient affinity of particles to thepolyester, but the present invention has made it possible to solve theseproblems by the use of the fine spherical silica particles and the newpolymer PENBB. PENBB films allow generally high F₅ values in both MD andTD direction than e.g. PET films while still fulfilling the aboveconditions of equations (7) and (8).

Known film production methods can be employed for making F₅ value in themachine direction 120 MPa or greater. For example, it can be achieved bya method in which chips of the PENBB composition are melt extruded intoa film and solidified to form an amorphous sheet and this sheet is thenbiaxially stretched in the machine and transverse directionssuccessively or simultaneously, followed by 1.05 to 2.0 fold stretchingin the machine direction and heat treatment. In this case, it ispossible to properly incorporate such treatments as heat setting beforerestretching in the machine direction, relaxing in the machine directionbefore restretching slight-degree stretching in the machine directionbefore or after restretching, etc. In the case of PENBB, it is possibleto obtain a film having an F₅ value of 150 MPa or greater in the machinedirection.

Concerning the physical properties of the film for magnetic recordingmedia, it is preferred that the film has, in addition to the abovecharacteristics, an average surface roughness (R_(a)), and a number ofbroken protuberances (Bp) on the film surface defined by the followingformulae (9) and (10).

    5≦R.sub.a ≦30(nm)                            (9)

    Bp≦10(per mm.sup.2)                                 (10)

When the average surface roughness is less than 5 nm, the film provespoor in handling properties, running characteristics and wearresistance, while when it exceeds 30 nm, the magnetic recording mediummade from the film is found to be poor in electro-magnetic transducingcharacteristics. The number of broken protuberances on the film surfaceis an indication of affinity of particles to PENBB. A greater number ofbroken protuberances indicates lower affinity. Bp values greater than10/mm² indicate poor affinity of particles, and in this case, largeamounts of powder are deposited on the calender rolls during theproduction of magnetic recording media, resulting in the deterioratedproperties of the products. Wear resistance of the magnetic recordingmedium at the time of recording and reproduction is also reduced.

Another preferable film property is that the number of coarseprotuberances from the film surface having a height of 0.81 μm orgreater, is less than 10 per 25 cm². This parameter is an especiallyimportant requirement for the base film of magnetic discs. This propertymaintains the good film handling characteristics in magnetic discproduction and can also enhance the electromagnetic characteristics ofthe produced magnetic disc.

It is also pointed out as still another preferable film property, thatthe average surface roughness (R_(a)) (nm) of the film, and the ratio ofmaximum height of protuberance (R_(t)) (nm) of the film to R_(a) (R_(t)/R_(a)) fall within the ranges defined by the following formulae (11)and (12), and that also the number of broken protuberances on the filmsurface is less than 20 mm² :

    3≦R.sub.a ≦15                                (11)

    5≦R.sub.t /R.sub.a ≦20                       (12)

When R_(a) is less than 3 nm, the film has unsatisfactory handlingcharacteristics and wear resistance, and when R_(a) exceeds 15 nm, therecording media produced from it are not satisfactory for a high-densityrecording. The R_(t) /R_(a) ratio is a measure of uniformity of theheight of protuberances. Generally, the smaller than R_(t) /R_(a) ratiois, the better is the uniformity. However, when the R_(t) /R_(a) ratiois less than 5, although the uniformity of protuberance height is good,the protuberances are too broad and the signal output from the medium isdecreased. When the R_(t) /R_(a) ratio exceeds 20, the protuberanceheight distribution becomes non-uniform and in this case, too, thesignal output decreases. The values of R_(a) and R_(t) /R_(a) are basedon average diameter of the particles contained, particle sizedistribution and particle content, but these values are also variableaccording to the base film producing conditions, that is, the stretchingconditions of the copolyester film made by a biaxial stretching method.In any case, the techniques employable in the present invention are notsubject to any specific limitations as far as they meet the requirementsof the present invention.

In the case that biaxially oriented PENBB films of the present inventionare used for capacitors, the volume resistivity of the starting PENBBcomposition in the molten state is preferably adjusted to 2.0×10⁸ Ω.cmor higher. When PENBB film is used for capacitors, the copolyester filmis required to satisfy some property requirements. One importantproperty requirement is that the film has a specific "CR value" which isthe product of electrostatic capacity and insulation resistance atnormal and nigh temperature. It has been found that this propertyrequirement can be met when the volume resistivity of the PENBBcomposition in the molten state is maintained at 2.0×10⁸ Ω.cm or above.

Maintenance of specific volume resistivity (P_(v)) at or above 2.0×10⁸Ω.cm can be achieved by adjusting the ratio of the phosphorus compoundused as thermal stabilizer to the alkaline earth metal compound used asester exchange catalyst (P/Me). Preferred examples of alkaline earthmetal compounds usable here are calcium acetate, magnesium acetate,manganese acetate and the like. Phosphoric acid and phosphoric acidesters can be favorably used as phosphorous compound.

Also, when the biaxially oriented PENBB film of the present invention isused for capacitors, it is preferable that the number of protuberanceson film surface is not less than 1.0×10³ per mm², and that thedifference (Δd) between micrometric film thickness (measured by using 10pieces of film) and gravimetric film thickness falls within the rangedefined by the following formula (13):

    0.1 μm≦Δd ≦0.4 μm                (13)

When the number of protuberances on the film surface is less than1.0×10³ per mm², slip characteristics of the film are bad and thereforefilm handling characteristics are poor.

Δd is a numerical indication of the degree of surface roughness (for themeasuring method, see the section of Examples described below). When thevalue of Δd exceeds 0.4 μm, the electrode gap in the produced capacitorbecomes too large, causing a reduction of electric capacity per unitvolume. When Δd is less than 0.1 μm, satisfactory slip characteristicscannot be obtained and the handling characteristics are poor.

The present invention will hereinafter be described in further detail inexamples. The determination methods and definitions of variousproperties and characteristics mentioned above are as described below.All references to amounts expressed as "parts" and "%" appearing in thefollowing Examples are based on weight unless otherwise noted.

(1) Average diameter and particle size distribution of fine sphericalsilica particles

The particle diameter was measured by electron microphotography, and theparticle size distribution was determined by measuring the diameters ofabout 1,000 particles and cumulating the numbers of the particlesaccording to diameter, beginning with the largest particle size. Thediameter of the particles when the cumulative amount thereof is 10% ofthe total number of particles was expressed as D₁₀ and the diameter ofthe particles when the cumulative amount thereof is 90% of the totalnumber of particles D₉₀, and the degree of breadth of particle sizedistribution was indicated by the value of D₁₀ /D₉₀, (diameterdistribution index).

Measurements of other additive particles were made by centrifugalprecipitation method.

(2) Volumetric shape parameter of fine spherical silica particles

In the present invention, volumetric shape parameter (φv)is defined asV/D³ wherein D is the maximum diameter (μm) of fine spherical silicaparticles as measured by electron microphotography and V is the measuredvolume (μm³) of the particles.

(3) Specific surface area ratio (R) of fine spherical silica particles##EQU7##

The specific surface area determined by BET method is the valuedetermined from the adsorption of nitrogen gas at the temperature ofliquid nitrogen.

(4) Inherent viscosity (IV) of polyester composition

0.1 g of PENBB composition was dissolved in 1 dl (=100 ml) of a 50/50(by weight) pentafluorophenol/hexafluoroisopropanol mixed solvent, andthe viscosity of the solution was measured at 25° C. The IV value isdetermined by the equation: ##EQU8## wherein ##EQU9## and

    c=polymer concentration.

(5) Particle dispersibility in film

It was determined by observing under a microscope the dispersed state ofparticles added in a biaxially stretched film.

(6) Volume resistivity of the PENBB composition in the molten state

12 g of PENBB composition was put into a test tube having a side armequipped with a stopcock for introduction of N₂ or evacuation and thendipped in an oil bath of 290° C. After the composition has beenperfectly melted, air bubbles were completely removed by repeatingevacuation and introduction of nitrogen gas. Then stainless steelelectrodes were inserted into the test tube, and after 10 min., a directcurrent of 3 kV was applied to the electrodes. Immediately after currentapplication, the current was read and the specific resistance wascalculated from the following formula: ##EQU10##

In the above formula, P_(v) is specific resistance (Ω.cm), I is current(A), S is cross-sectional area (cm²) Of the electrode, and l is thedistance (cm) between the electrodes.

(7) Surface roughness R_(a)

The R_(a) value gives the surface roughness in the form of thearithmetic average of all displacements of the roughness R from thecenter line. The measurement is performed according to DIN standard 4768on a surface tester (Perthometer SPG made by Feinpruf GmbH of Gottingen,Germany). The values given are based on 10 individual measurements, 5 inMD and 5 in TD, in which the highest value is discarded beforeaveraging. The cut-off, i.e. individual length of the measurementsections, is 0.08 mm, the stylus radius 2 μm and 30 mg.

(8) Maximum height of protuberance

Maximum height of protuberance was expressed by the difference betweenthe maximum value and the minimum value of the sectional curve of filmobtained in the determination of surface roughness described in thepreceding section. R_(t) shown here is the average of the measurementsat total 10 points as in the case of Ra.

(9) Number of coarse protuberances.

Aluminum was deposited on the film surface and the sample examined witha double beam interference microscope. The protuberances showing the 2ndorder interference fringes at a measuring wavelength of 0.54 μm (H²)were inferred to have a height between 0.54 and 0.81 μm, and theprotuberances showing interference fringes of 3rd and higher orders (H³)were inferred to have a height of 0.81 μm or greater. The number ofthese protuberances was counted per 25 cm².

(10) Number of surface protuberances

The number of protuberances per 0.1 mm² was counted from lightmicrophotographs of film surface and converted to the number per 1 mm².

(11) Degree of surface roughness, Δd

a) Micrometric film thickness measured with 10 pieces of film placed oneon another is indicated by d₁₀. The thickness of a pile of 10 pieces offilm was measured by using a 1/1000 micrometer with a measuring force of700±100 gf and a measuring length of 0-25 mm, and the obtained value wasdivided by 10 to give the d₁₀ value (μm).

b) Gravimetric film thickness is indicated by d_(w). A 10 cm squarepiece was cut out from the film and its weight W was measured by Mettlerelectron balance, and d_(w) was determined from the following formula:##EQU11##

c) Δd was defined as follows:

    Δd=d.sub.10 -d.sub.w (μm)

(12) Mechanical properties

The tensile modulus, tensile strength, elongation at break and F₅ value(stress at 5% elongation) were determined in a tensile tester made byZwick (Ulm, Germany) under an atmosphere of 23° C. and 50% RH under thefollowing conditions:

Specimen shape: strip film (15 cm long and 15 mm wide)

Initial chuck interval: 100 mm

Draw rate: 10%/min for modulus 100% /min for other values

(13) Number of broken protuberances

The gold-deposited film surface was photographed at a magnification of2000 by a scanning electron microscope, and the number of thedepressions or dents made after disappearance of protuberances believedto be formed by particles was counted and converted to the number perunit surface area. Smaller values are desirable.

EXAMPLE 1

Synthesis of silica particles

30.4 g of tetramethylsilane is dissolved in 297 g of methanol andmaintained at 20° C. (solution A). Separately, 95 g of water is added to878 g of methanol and this solution is mixed with 243 g of a 28% aqueousammonia solution and maintained at 20° C. (solution B). The solution Ais added to solution B under stirring. Upon addition, hydrolysis andcondensation take place, and the reaction system becomes cloudy. Afterthe addition of solution A, the mixed solution is further stirred forone hour, after which 288 g of ethylene glycol ae added. The mixture isthen heated under reduced pressure to distill off excess water, methanoland ammonia, obtaining an ethylene glycol slurry containing silica in aconcentration of 4%. After drying this ethylene glycol slurry, theaverage particle diameter is determined by electron microphotograph. Theaverage particle diameter is 0.15 μm and D₁₀ /D₉₀ is 1.59. Uniformlyspherical particles with extremely high uniformity in size are obtained.The slurry is filtered using a 3 μm filter. The slurry shows excellentfiltering characteristics.

EXAMPLE 2

PENBB COMPOSITION

289 parts by weight of dimethyl 2,6-naphthalene dicarboxylate, 322 partsby weight of dimethyl, 4,4'-bibenzoate, 386 parts by weight of ethyleneglycol and 0.7 parts of manganese acetate tetrahydrate are initiallyintroduced into a conventional polycondensation reactor provided with ablanketing gas line (and N₂), pressure equalization, a thermometer, acondenser, a vacuum connection and a stirrer. The mixture was heated at220° C. for 2.5 hours, during which time methanol was distilled off.0.675 parts by weight of triphenyl phosphate, 0.2259 parts of antimonytrioxide and 57 parts of the above silica slurry (at a concentration of4%) are then added and the mixture is heated to 270° C., with stirring.Vacuum is applied and the temperature is raised to 285° C. andmaintained for 2.5 hours. A portion of the copolyester thus obtained isejected from the reactor in the form of melt as a bubble under nitrogenpressure and solidified as a thin, clear, amorphous film. A glasstransition temperature (T_(g)) of 123° C. and a cold crystallizationtemperature (T_(cc)) of 132° C. is determined for the film by means ofDSC. The residual melt is then granulated. These PENBB granules, havinga melting point of 281° C. are melted in a single screw extruder attemperatures of 280°-320° C. and extruded through a sheet dye unto acooling roll temperature controlled at 30° C.

A 120 μm thick film is obtained which is clear and transparent. ThisPENBB pre-film is then sequentially biaxially oriented (firsttransversely, then longitudinally: 3.5×3.5) at 130° C. on a filmstretching device. An 8 μm thick, clear film is obtained. The film isfinally heat treated at 260° C. for 10 minutes. The film showedexcellent coefficient of friction, low number of coarse particles, goodwinding characteristics and wear resistance. Compared to similarlyprepared PET films, the mechanical properties of the resultant PENBBfilm are much better, as shown in Table I.

                  TABLE I                                                         ______________________________________                                        Property      Units    PENBB         PET                                      ______________________________________                                        Tensile Strength                                                              MD            (MPa)    240                                                    TD                     180           100-200                                  Elongation at Break                                                           MD            (%)      25                                                     TD                     20            50                                       F.sub.5 value                                                                 MD            (MPa)    172                                                    TD                     151           100-130                                  Tensile Modulus                                                               MD            (GPa)    9.2                                                    TD                     8.1           4-6                                      Shrinkage (at 150° C. for                                              15 minutes)                                                                   MD            (%)      0.3           >1.0                                     TD                     0.3           >1.0                                     ______________________________________                                    

That what is claimed is:
 1. A copolyester composition comprisingpoly(ethylene naphthalate bibenzoate) and 0.001 to 5% by weight of afine spherical silica particles having an average diameter of 0.01 to3.0 μm.
 2. Copolyester composition according to claim 1, wherein saidfine spherical silica particles are obtained by subjecting alkoxysilaneto a hydrolysis and condensation reaction.
 3. The copolyestercomposition according to claim 2, wherein said fine spherical silicaparticles comprises 90% by weight or more of silicon dioxide.
 4. Acopolyester composition according to claim 1 wherein the diameterdistribution index of said fine spherical silica particles, representedby the formula (1), is in the range of 1.1 to 2.7: ##EQU12## wherein D₁₀is the diameter of the particle when the cumulative number thereof is10% of the total number of said particles, D₉₀ is the diameter of theparticle when the cumulative number thereof is 90% of the total numberof said particles and D₁₀ and D₉₀ are measured under an electronmicroscope, said cumulative numbers being calculated beginning with thelargest particle size.
 5. The copolyester composition according toclaims 1 or 5 wherein the fine spherical silica particles have avolumetric shape parameter represented by the following formula 2:##EQU13## wherein φv is defined as φv=V/D³, wherein V is the volume ofthe particle (μm³) and D is the maximum projected diameter (μm) of theprojection of the particle.
 6. The copolyester composition according toclaims 1 or 4, wherein said fine spherical silica particles have aspecific surface area ratio defined by the following formula (3) of 5.0or above: ##EQU14##
 7. The copolyester composition according to claim 1which comprises a combination of fine spherical silica particles (A) andinactive particles (B) other than particles (A).
 8. The copolyestercomposition according to claim 7 wherein the particles (B) are particles(B-I) selected from the group consisting of silica particles other thanthe fine particles (A), calcium carbonate, amorphous zeolite, titaniumdioxide, calcium phosphate, kaolin, talc, clay and mixtures thereof. 9.The copolyester composition according to claim 8 which contains inaddition to the fine particles (A), 0.005 to 1.0% by weight of theparticles (B-I) having a D₂ /D₁ ratio defined by the following formulain the range of 1.1 to 3.0: ##EQU15##
 10. The copolyester compositionaccording to claim 9, wherein the Moh's hardness of said particles (B-I)is 4.0 or below.
 11. The copolyester composition according to claim 7wherein the particles (B) are particles (B-II) selected from the groupconsisting of fine particles comprising calcium, lithium and phosphorouscompound, fine particles comprising calcium and phosphorous compoundsand fine particles comprising calcium, magnesium and phosphorouscompounds, said fine particles having a diameter of about 0.1 to 3 μm.12. The copolyester composition according to claim 11 which contains theparticles (B-II) in an amount of 0.05 to 1.0% by weight based on thepoly(ethylene naphthalate bibenzoate).
 13. The copolyester compositionaccording to claim 11 or 12, wherein the particles (B-II) are particlescontaining the elements calcium, lithium and phosphorous each in anamount of 1.0% by weight or more based on the weight of said particles(B-II).
 14. The biaxially oriented copolyester film according to any oneof claims 1, 4, 7, 8, 9, 10, 11, 12, wherein the film surface roughness(R_(a)) and the number of broken protuberances per 1 mm² (Bp) on thefilm surface satisfy the following formulae (9) and (10):

    5≦R.sub.a ≦30 (nm)                           (9)

    Bp≦10                                               (10).


15. A biaxially oriented copolyester film comprising the copolyestercomposition according to claims 1 or
 7. 16. The biaxially orientedcopolyester film according to claim 15, wherein the number ofinterference fringes of n-th order (H_(n) per 1 cm²) as measured bydouble beam interference method, satisfies the following formulae (5 and(6): ##EQU16##
 17. The biaxially oriented copolyester film according toclaim 16 wherein the F₅ value of the film in the machine direction is120 MPa or above and the birefringence is less than 0.2 and the inherentviscosity of the poly(ethylene naphthalate bibenzoate) is greater than0.5 dl/g.
 18. The biaxially oriented copolyester film according to claim17 wherein the film surface roughness (R_(a)) and the number of brokenprotuberances per 1 mm² (Bp) on the film surface satisfy the followingformulae (9) and (10)

    5≦R.sub.a ≦30 (nm)                           (9)

    Bp≦10                                               (10)

and the number of coarse protuberances having a height of 0.81 μm orgreater on the film surface is 10 or less per 25 cm² and the number ofcoarse protuberances having a height of 0.54 μm to less than 0.81 um is50 or less per 25 cm² and wherein the diameter distribution index of theparticles (A) represented by the formula (1) is in the range of 1.1 to2.7 ##EQU17## wherein D₁₀ is the diameter of the particle when thecumulative number thereof is 10% of the total number of said particles,D₉₀ is the diameter of the particle when the cumulative number thereofis 90% of the total number of said particles and D₁₀ and D₉₀ aremeasured under an electron microscope, said cumulative numbers beingcalculated beginning with the largest particle size.
 19. The biaxiallyoriented copolyester film according to claim 15, wherein the F₅ value ofthe film in the machine direction is 120 MPa or above.
 20. The biaxiallyoriented copolyester film according to claim 15, wherein thebirefringence is less than 0.2 and the inherent viscosity of thepoly(ethylene naphthalate bibenzoate) is greater that 0.5 dl/g.
 21. Thebiaxially oriented copolyester film according to claims 19 or 20 whereinthe number of coarse protuberances having a height of 0.81 μm or greateron the film surface is 10 or less per 25 cm² and the number of coarseprotuberances having a height of 0.54 μm to less than 0.81 μm is 50 orless per 25 cm².
 22. The biaxially oriented copolyester film accordingto claims 19 or 20 wherein the average surface roughness R_(a) (nm) ofthe film and the ratio of the maximum height of protuberance R_(t) (nm)of the film to R_(a) satisfy the following formulae (11) and (12),respectively, and the number of broken protuberances on the film surfaceis 20 or less per mm² :

    3≦R.sub.a ≦15                                (11)

    5≦R.sub.t /R.sub.a ≦20                       (12).


23. The biaxially oriented copolyester film according to claims 19 or 20wherein said film has a thickness of 4 to 30 μm.
 24. The biaxiallyoriented copolyester film according to claims 19 or 20 wherein said filmhas a thickness of 30 to 100 μm.